Within a relatively brief time, semiconductor materials have made possible the creation of a wide range of optical and electronic devices which have played a major role in the shaping of our world. The impact of semiconductor devices has been felt from the battlefield to the playground and from the kitchen to the cosmos. In the earliest stages, semiconductor technology was limited by the use of single crystalline materials. These materials were, of necessity, highly pure and possessed a morphology with extremely regular and long-range periodicity. The dual and interdependent constraints of periodicity and stoichiometry restricted the compositional range, and hence physical properties of crystalline semiconductor materials. As a result single crystalline devices were expensive, difficult to fabricate and limited in their properties.
While conventional wisdom at the time dictated that semiconductor behavior could only be manifested in highly ordered materials, it was recognized by S. R. Ovshinsky that the requirements of periodicity could be overcome and that semiconductor behavior is manifested by various disordered materials. In this regard, see "Reversible Electrical Switching Phenomena and Disordered Structures" by Stanford R. Ovshinsky; Physical Review Letters, vol. 21, No. 20, Nov. 11, 1968, 1450 (C) and "Simple Band Model for Amorphous Semiconducting Alloys" by Morrel H. Cohen, H. Fritzsche and S. R. Ovshinsky; Physical Review Letters, vol. 22, No. 20, May 19, 1969, 1065 (C). Disordered materials are characterized by a lack of long-range periodicity. In disordered semiconductors the constraints of periodicity and stoichiometry are removed and as a result, it is now possible to place atoms in three dimensional configurations previously prohibited by the lattice constants of crystalline materials. Thus, a whole new spectrum of semiconductor materials having novel physical, chemical and electrical properties has been made available. By choice of appropriate material compositions, the properties of disordered semiconductors may be custom tailored over a wide range of values. Disordered semiconductors may be deposited by thin film techniques over relatively large areas and at low cost, and as a result many types of new semiconductor devices have become commercially feasible. A first group of disordered semiconductors are generally equivalent to their crystalline counterparts while a second group manifest physical properties that cannot be achieved with crystalline materials.
As a result of the foregoing, disordered semiconductor materials have come to be widely accepted and a great number of devices manufactured therefrom are in significant commercial use. For example, large area photovoltaic devices are routinely manufactured from amorphous silicon and germanium-based alloys. Such materials and devices are disclosed, for example, in U.S. Pat. Nos. 4,226,898 and 4,217,374 of Ovshinsky et al. Disordered alloy materials have also been used to fabricate photodetector arrays for use in document scanners, drivers for LCD displays, cameras and the like. In this regard see U.S. Pat. No. 4,788,594 of Ovshinsky et al. Disordered semiconductor materials have also been used in devices for the high volume storage of optical and electronic data.
Amorphous materials are presently utilized in a manner to take advantage of the great variety of interactions between constituent atoms or molecules in contrast to the restricted number and kinds of interactions imposed by a crystalline lattice. In the present invention, the advantages of crystalline and amorphous properties can be combined for those devices and applications in which periodicity is essential to the physics. Periodicity can be placed in an amorphous matrix through the utilization of the present invention. The material can include spatially repeating compositional units, atoms, groups of atoms or layers without the overall bulk inhibition of crystalline periodicity.
Also, individual atoms or groups of atoms in various configurations can be provided, which can be combined with other atoms or groups of atoms and be disbursed throughout the material. As stated, the individual atoms or groups of atoms, in these materials need not be in a regular pattern, but can have a varying spatial pattern, such as being graded or nonsequential throughout the material. By the proper choice of atoms or groups of atoms, their orbitals and isolated configurations, anisotropic effects not permitted in any prior type of material can be produced.
These procedures provide varying geometrical environments for the same atom or a variety of atoms, so that these atoms can bond with other surrounding atoms in different coordination configurations as well as unusual nonbonding relationships resulting in entirely new chemical environments. The procedures provide means for arranging different chemical environments which can be distributed and located throughout the material in the spatial pattern desired. For example, one part or portion of a material can have entirely different local environments from other portions. The varying electronic states resulting from the various spatial patterns which are formed and the various chemical environments which can be designed, can be reflected in many parameters as a type of density of states or change of states in the energy gap of a semiconductor except that this density of states can be spatially arranged.
In essence, the material of the invention is a compositionally modulated material utilizing the very concept of irregularity, inhomogeniety, "disorder" or localized order which have been avoided in the prior art, to achieve benefits which have not been exhibited in prior materials. The local environments need not be repeated throughout the material in a periodic manner as in the compositionally modulated materials of the prior art. Further, because of the above-described effects the specific types of disorder and their arrangement in a spatial pattern, the materials as described by this invention cannot be thought of as truly amorphous materials as typically produced by the prior art since the material is more than a random placement of atoms. The placement of atoms and orbitals of a specific type that can either interact with their local environment or with one another depending upon their spacing throughout an amorphous material and an amorphous matrix can be achieved. The composite material appears to be homogeneous, but the positions of the orbitals of the atoms can have relationships designed to emphasize a particular parameter, such as spin compensation or decompensation. The materials thus formed give a new meaning to disorder based on not only nearest neighbor relationships, but "disorder" among functional groups, which can be layers or groups, on a distance scale which can be as small as a single atomic diameter. Hence, a totally new class of "synthetic nonequilibrium multi-disordered" materials have been made available.
It has been found that properties of semiconductor materials in the disordered state will depend upon their morphology and local chemical order and can be affected by various methods of preparation. For example, non-equilibrium manufacturing techniques can provide a local order and/or morphology different from that achieved with equilibrium techniques; and as a result, can change the physical properties of the material. In most instances, an amorphous semiconductor will have a lower electrical conductivity than the corresponding crystalline material and in many instances, the band gap energy, optical absorption coefficient and electronic activation energy of corresponding amorphous and crystalline materials will differ. For example, it has been found that amorphous silicon materials typically have a band gap of approximately 1.6-1.8 eV while crystalline silicon has a band gap of 1.1 eV. It is also important to note that amorphous silicon materials have a direct band gap while the corresponding crystalline material has an indirect band gap and as a result, the optical absorption of amorphous silicon is significantly higher than that of crystalline silicon at or near the band edge. It should also be noted that the dark electrical conductivity of undoped amorphous silicon is several orders of magnitude lower than that of crystalline silicon. It can thus be seen that the various physical properties of silicon strongly depend upon its morphology and local order. Similar relationships are found in a large number of other semiconductor materials.
The principle of the present invention resides in the ability to control the local order of a semi-conductor material from that corresponding to a completely amorphous phase through various other local organizations including intermediate order to a state where the local order is so repetitively periodic that the material is in the single crystalline state. The most important and interesting area of the present invention is the ability conferred thereby to control the local order of a semi-conductor material to produce a material which has valuable properties different from either the amorphous or the crystalline states.
The various properties of amorphous and crystalline silicon confer different advantages in various devices. The high mobility of carriers in crystalline silicon is important in high speed semiconductor circuits while the high level of optical absorption of amorphous silicon is ideal for photovoltaic devices since complete light absorption may be accomplished by relatively thin layers of material, making for a lightweight, low cost device. In some instances, one property of a given morphology and local order of semiconductor may be ideal for a particular purpose whereas the value of another property of that same material may not be so well suited. For example, the aforenoted high optical absorption of amorphous silicon is ideal for a photovoltaic device; however, the fairly wide band gap of amorphous silicon does not permit it to address the longer wavelength portions of the solar spectrum. The use of narrower band gap crystalline material in photovoltaic devices increases the portion of the useable light spectrum and the high conductivity, high mobility and long minority carrier diffusion length in crystalline silicon decreases the series resistance of the photovoltaic device, thereby increasing its overall efficiency; but, the trade-off is that crystalline cells are relatively thick because of their low absorption and hence they are fragile, bulky and expensive.
Previously Ovshinsky, et al. produced materials which included clusters of atoms, typically between 12 and 50 angstroms in diameter. See U.S. Pat. No. 5,103,284, issued Apr. 7, 1992 and entitled "SEMICONDUCTOR WITH ORDERED CLUSTERS." The clusters or grains had a degree of order which is different from both the crystalline and amorphous forms of the material. The small size and ordering of the clusters allowed them to adjust their band structure to thereby relax K vector selection rules. Ovshinsky et al had found that various physical properties of semiconductor materials are decoupled from morphology and local order when those materials are comprised of ordered clusters. This selection rule relaxation occurred because the materials included a volume fraction of the intermediate order materials which was high enough to form percolation pathways within the material.
The onset of the critical threshold value for the substantial change in the physical properties of materials in the ordered cluster state depends upon the size, shape and orientation of the particular clusters. However, it is relatively constant for different types of materials. There exist 1-D, 2-D and 3-D models which predict the volume fraction of clusters necessary to reach the threshold value, and these models are dependent on the shape of the ordered clusters. For example, in a 1-D model (which may be analogized to the flow of charge carrier through a thin wire) the volume fractions of clusters in the matrix must be 100% to reach the threshold value. In the 2-D model (which may be viewed as substantially conically shaped clusters extending through the thickness of the matrix) the volume fraction must be about 45% to reach the threshold value, and finally the 3-D model (which may be viewed as substantially spherical clusters in a sea of matrix material) the volume fraction need only be about 16-19% to reach the threshold value.
Therefore, the materials disclosed and claimed in U.S. Pat. No. 5,103,284 have at least 16-19 volume percent of intermediate range order material for spherical clusters, at least 45 volume percent for conically shaped clusters and 100 volume percent for filamentary clusters.
The instant inventors have now found that materials including any volume percent of the intermediate range order material (i.e. the ordered clusters) will have properties which (while not necessarily decoupled) differ from materials with no intermediate range order material. These materials are particularly useful in the form of thin films used in devices such as: photovoltaic devices, thin-film diodes, thin-film transistors, photoreceptors, etc.